In recent years, various kinds of portable electronic devices are developed and batteries are used as power sources of these electronic devices. For the portable electronic devices, compact and light forms are demanded. For the batteries employed for these devices, compact and light forms are also demanded.
In the electronic devices including the portable electronic devices, the batteries high in their degree of freedom in forms are required so that accommodation spaces may be efficiently used. As a battery which satisfies such a demand, a lithium secondary battery high in its energy density and output density or a lithium-ion secondary battery using a carbon material capable of reversibly doping and dedoping lithium ions for an anode are proposed. Particularly, the lithium-ion secondary battery has been widely put to practical use and widely employed as power sources of portable electronic devices such as camcorders, note book type personal computers, portable telephones, etc.
The lithium secondary battery and the lithium-ion secondary battery have a problem that a discharging capacity is gradually reduced due to the repetition of charging and discharging operations. This phenomenon may sometimes result from the deterioration of performances of materials forming the battery such as a cathode material, a conductive agent, a binding agent, an electrolyte, etc., however, this phenomenon substantially results from a reaction of an anode and an electrolyte. Since the anode serves as a powerful reducing agent under its charged state, the anode is extremely apt to react with the electrolyte. When the anode in its charged state reacts with the electrolyte, the anode is considered to generate a self-discharge. As a result, a difference in depth of charge arises between the anode and the cathode. Since either the cathode or the anode cannot be charged and discharged in the battery, the difference in depth of charge between the anode and the cathode causes the decrease of capacity that cannot be recovered. When the anode reacts with the electrolyte, the anode may be denatured or disintegrated to be deactivated so that the capacity may be possibly decreased.
The above-described reaction of the anode with the electrolyte is suppressed by a passive film formed on the surface of the anode due to a reductive decomposition of the electrolyte upon charging. The formation of the film is accompanied with the self-discharge of the anode as described above. The formed film likewise increases the internal resistance of the battery. That is, when the film is excessively formed, the energy density and the output density are deteriorated. On the contrary, when the film is insufficiently formed, the deterioration of charging and discharging cyclic characteristics cannot be adequately suppressed. Accordingly, in order to make the high energy density and the output density compatible with the excellent charging and discharging cyclic characteristics, a film of good quality needs to be formed as low as possible as required.